Kotlin 2.3.0 Release Candidate (RC) is now available, bringing several notable features and improvements. Among the highlights are a new experimental checker for unused return values, enhanced interoperability with Swift, support for Java 25, and various updates for Kotlin/Wasm and Kotlin/JS. Developers can access the Kotlin 2.3.0-RC on GitHub, with the stable release expected next month or in January 2026.

New Unused Return Value Checker

The latest release introduces an experimental checker that warns developers when a function’s return value is ignored. This feature helps catch bugs where important results are silently discarded, especially when functions return meaningful data other than Unit or Nothing. The checker activates during static analysis and aims to improve code safety and correctness.

As an experimental feature, it encourages developers to review and refine their code to prevent potential silent bugs related to ignored return values. This addition underscores Kotlin’s focus on enhancing code reliability and developer tools.

Additional Features and Improvements

Two language features previously in beta have now become stable: support for nested type aliases and data flow exhaustiveness checks in when expressions. Kotlin also improves context-sensitive resolution by considering sealed and super types in scope and warning about ambiguities when using type operators and equalities.

Interoperability with Swift has been enhanced through Kotlin/Native, now supporting native enum classes and variadic function parameters. Previously, Kotlin enums exported to Swift appeared as classes; now, they map directly to native Swift enums, making integration more seamless.

Java support has been updated to generate classes compatible with Java 25 bytecode, aligning Kotlin with the latest Java standards released in September. The compiler remains fully compatible with Gradle versions 7.6.3 through 9.0.0, though developers should be mindful of potential deprecation warnings with newer Gradle versions.

WebAssembly and JavaScript Enhancements

For Kotlin/Wasm, the KClass.qualifiedName property is enabled by default, allowing runtime access to fully qualified names without additional configuration. This change does not increase binary size thanks to compiler optimizations. Additionally, support for a new WebAssembly exception handling proposal is enabled by default for wasmWasi targets, improving runtime error handling.

In Kotlin/JS, transpilation features continue to evolve, enhancing compatibility and performance for JavaScript environments. Overall, these updates aim to make Kotlin more robust and versatile across different platforms.

Inspired by

• https://www.infoworld.com/article/4096340/kotlin-introduces-checker-for-unused-return-values.html

Sources

• kotlinlang.org
• github.com
• github.com